“They will send their sinister army armed to the teeth to fight us, assaulting the working class, the peasants and popular masses, spreading their sinister claws. They will try to encircle, isolate, crush and wipe us out. But we are the future, the strength, and history. Comrades, revolution and counterrevolution are also contending forces in our country. They are two parts of a unity of opposites in constant struggle. The reactionaries are armed and concentrated, defending the metropolis and capitals. We are rooted in the countryside, in small villages, with the masses, especially with the poor peasants, with the force of the people, among their disorganized force in order to organize it into a powerful army. But this will not be easy.”

Book XIII
Homer His Iliads Translated (1660)

Source: Terrorism and Communism (1920), Ch. 3

This is also sometimes quoted as "The millions of people, armed in the holy cause of liberty".
1770s, "Give me liberty, or give me death!" (1775)
Context: They tell us, sir, that we are weak; unable to cope with so formidable an adversary. But when shall we be stronger? Will it be the next week, or the next year? Will it be when we are totally disarmed, and when a British guard shall be stationed in every house? Shall we gather strength by irresolution and inaction? Shall we acquire the means of effectual resistance by lying supinely on our backs and hugging the delusive phantom of hope, until our enemies shall have bound us hand and foot?
Sir, we are not weak if we make a proper use of those means which the God of nature hath placed in our power. Three millions of people, armed in the holy cause of Liberty, and in such a country as that which we possess, are invincible by any force which our enemy can send against us. Besides, sir, we shall not fight our battles alone. There is a just God who presides over the destinies of nations, and who will raise up friends to fight our battles for us. The battle, sir, is not to the strong alone; it is to the vigilant, the active, the brave.

Source: Revolutionary Suicide (1973), p. 3

2013, Second Inaugural Address (January 2013)
Context: We will defend our people and uphold our values through strength of arms and rule of law.  We will show the courage to try and resolve our differences with other nations peacefully –- not because we are naïve about the dangers we face, but because engagement can more durably lift suspicion and fear. America will remain the anchor of strong alliances in every corner of the globe.  And we will renew those institutions that extend our capacity to manage crisis abroad, for no one has a greater stake in a peaceful world than its most powerful nation. We will support democracy from Asia to Africa, from the Americas to the Middle East, because our interests and our conscience compel us to act on behalf of those who long for freedom.  And we must be a source of hope to the poor, the sick, the marginalized, the victims of prejudice –- not out of mere charity, but because peace in our time requires the constant advance of those principles that our common creed describes: tolerance and opportunity, human dignity and justice.

Source: Guerrilla Warfare (1961), Ch. I: 1. Essence of Guerrilla Warfare

Listen Yankee (1960), pp. 54.
1960s

"The Politics of Mass Strikes and Unions"; Collected Works 2 <!-- p. 465 -->
Context: The modern proletarian class doesn't carry out its struggle according to a plan set out in some book or theory; the modern workers' struggle is a part of history, a part of social progress, and in the middle of history, in the middle of progress, in the middle of the fight, we learn how we must fight... That's exactly what is laudable about it, that's exactly why this colossal piece of culture, within the modern workers' movement, is epoch-defining: that the great masses of the working people first forge from their own consciousness, from their own belief, and even from their own understanding the weapons of their own liberation.